Containers are presently in use in which bulk granular material is transported and stored. These containers are generally provided with a discharge mechanism at their lower end through which material is discharged into a hopper or other receptacle. In one type of container, the discharge mechanism consists chiefly of a sliding door which is slid from under the stored material to allow it to drop out of the container, and is subsequently slid inwardly to stop the downward flow. Such a container is disclosed in the patent to Coleman U.S. Pat. No. 3,354,918 in which the sliding door is moved inwardly and outwardly of the container through a slot in one side wall of the container. When the door is in its inward position, it is raised bodily upwardly a short distance until its upper peripheral edge surface engages a sealing gasket on the container. This type of sliding door discharge mechanism is generally satisfactory but it has been found that, as the door is slid outwardly during the discharging operation, dust is forced upwardly from the receiving hopper and expelled out of the container through the slot into the working area. Some efforts have been made to overcome this problem. For example, the Coleman U.S. Pat. No. 3,704,722 discloses a mechanism wherein a cam-actuated bracket is raised across the slot into sealing engagement with a gasket to close the slot when material is to be discharged from the container. Borger U.S. Pat. No. 3,182,954 discloses a mechanism for sliding the door to open position which includes a plate that allegedly fills the slot. The present invention provides a new mechanism for preventing undesirable discharge of dust into the working area, namely a butterfly valve that is effective to hold material in the container until the sliding door has been completely opened and the slot has been sealed off from communication with the discharge passage. It is therefore an object of the present invention to provide an improved discharge machanism which will permit the discharge of material while substantially eliminating the escape of dust into the surrounding atmosphere.